Computer disk drives store information on disks or platters. Typically, the information is stored on each disk in concentric tracks. The data tracks are usually divided into sectors. Information is written to and read from a storage surface(s) of a disk by a transducer. The transducer may include a read element separate from a write element, or the read and write elements may be integrated into a single read/write element. The transducer is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different data tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer to access different sectors within each track on the disk.
The actuator arm is coupled to a motor or coarse actuator, such as a voice coil motor (VCM), to move the actuator arm such that the transducer moves radially over the disk. Operation of the coarse actuator is controlled by a servo control system. The servo control system generally performs two distinct functions: seek control and track following. The seek control function includes controllably moving the actuator arm such that the transducer is moved from an initial position to a target track position. In general, the seek function is initiated when a host computer associated with the computer disk drive issues a command to read data from or write data to a target track on the disk. Once the transducer has been moved sufficiently close to the target track by the seek function of the control system, the track following function of the servo control system is activated to center and maintain the transducer on the target track until the desired data transfers are completed.
The track following function of the servo control system generally includes maintaining the transducer at a desired position with respect to a track being followed (e.g., over a centerline of the track). Typically, the transducer must be moved slightly during track following to maintain a desired position over the track. This is because, due to various factors, the track may appear to move beneath the transducer. There are numerous outside influences which can make it difficult for the servo control system to maintain the desired position over the track, one such influence is known as “runout.”
Runout generally refers to deviation from perfect circular motion and, more particularly, refers to variation in the distance between an external point of reference and a passing surface of a rotating object. “Repeatable runout” involves periodic deviations that occur with predictable regularity (hereafter “RRO”). “Nonrepeatable runout” involves random perturbations due, for example, to bearing slop, shock events, and so on (hereafter NRRO). In the context of a disk drive, RRO is “repeatable” because it occurs in sync with the spinning disk. RRO may be caused by one or more of the following mechanical sources: a) spindle motor runout; b) disk slippage; c) disk warping; d) disturbances converted to RRO during a servo writing process due to, for example, NRRO, vibrations, resonances, media defects, or disk distortion due to clamping of the HDA. RRO may also be caused by electromagnetic imperfections due to low quality servo positioning bursts, even if they were mechanically recorded on the ideal circle. Such low quality servo positioning bursts can yield incorrect position information.
The ability to precisely position a transducer with respect to a track being followed has become increasingly important as data and track densities in hard disk drives have increased. Adaptive runout cancellation may be used during track following to feed-forward a control signal to the servo control system to continually move the transducer so as to compensate for the time varying effect of RRO. However, continual movement of the transducer during track following can consume a substantial amount of power in the disk drive, which may be unacceptable when the disk drive is within a battery powered device.